The present invention relates to an improved process for manufacturing radial and bias/belted tires which eliminates the use of undertread cement.
In the tire industry the tack of uncured rubber compounds has always been one of the most important properties required for building tires. Tack may be defined as the ability of two uncured rubber materials to resist separation after bringing their surfaces into contact for a short time under a light pressure.
It is important that the components of the tire exhibit quick-grab tack when building the tire; and the tack bonds should have long term resistance to separation, because the green tire may be hung on a rack for a few days before vulcanization. In addition the bonded portions of the uncured tire must have adequate green strength so that there is no excess distortion or creep before curing and no tear during the expansion that occurs upon molding (or in the second stage for a radial tire).
In factory tire manufacturing operations lack of adequate building tack has always been a problem and undertread cements and/or solvents have always been used to assure adequate tack during building of a green tire and to minimize the number of defective tires. The use of hydrocarbon solvents at the tire building machine should, of course, be avoided because of excessive cost and the fire and health hazards.
For many decades the tire industry has employed tread cements compounded to provide optimum tack and tack retention characteristics and has considered them essential to assure safe, anomaly-free tires and to maintain adequate safety standards. The need for tread cements was critical, particularly during the last decade in the manufacture of modern belted radial tires designed to operate safely at very high speeds under the severest conditions.
Even when using the best undertread cements, tire manufacturers have had frequent problems in assuring adequate building tack and have had to scrap large numbers of treads or green tires due to tire building problems and defective bonds between the undertread and the underlying belt compound and/or sidewall stock. In steel-belted radial passenger tires, for example, building tack is a particularly serious matter because of the nature of the belt compound in which the wires are embedded. Such belt compound has notoriously poor tack because of its stiffness (i,e., a 300% modulus of about 2300 psi), the relatively large amount of HAF carbon black and the relatively small amount of oil.
Safety standards with respect to radial and bias/belted tires have been continually increasing over the years, and extensive research has been carried on during the last decade in an attempt to reduce the number of tires with defects in the vicinity of the undertread and to improve the reliability of the adhesive bond between the undertread and the belt compound and/or the sidewall stock. The tire industry has made a strong effort to produce radial and bias/belted truck and passenger car tires at reasonable cost while maintaining high safety and quality standards.
Tire manufacturers have adopted generally standard and well-accepted procedures for mass production of tires which involve use of dual-layer extruded treads having undertreads which are adhered by an undertread cement to the high-modulus belt coat compounds. These factory procedures require good tack and tack retention to permit reliable economical tire building. Therefore, it has been absolutely necessary to employ undertread cements.
Modern radial and bias/belted passenger car tires must be designed to provide good crack and abrasion resistance, low hysteresis, low rolling resistance, and good gas mileage as well as long service life and therefore employ blends of SBR and BR rubber in both the tread cap and the undertread. A typical extruded undertread produced by standard prior art factory processes and containing about 50 phr of BR rubber and about 30 phr of SBR rubber has inadequate tack to permit tire building in the absence of undertread cements, even when compounded to provide optimum tack. This is one reason that it has heretofore been impossible to mass produce belted radial passenger car tires without employing undertread cements to provide the essential building tack.
The exposed surface of an undertread (to which the cement is applied) normally contains a wide variety of different compounds which can have an adverse effect on tack and tack retention. These include sulfur; zinc stearate, calcium stearate and other calcium and zinc salts; stearic acid, palmitic acid and other unreacted fatty acids; antioxidants; vulcanization accelerators; retarders; waxes,; etc. These also include compounds produced or modified by oxidation or ozone attack on the surface (see Rubber Chem & Tech, 52 823, 1979). The compounding ingredients which are not dissolved in the rubber tend to diffuse and migrate to the surface so that their concentration at the surface is much higher than that of the interior rubber portions. Oxidation, degradation, bloom and contamination affect tack and adhesion and are typical of problems encountered in tire building. They involve many different reactions which are not fully understood by rubber compounders, but these problems and the problems of wax bloom and sulfur bloom have confronted the art for over 30 years, and the normal factory procedures have for many years controlled the problem well enough to permit economical mass production of belted radial passenger tires of the highest quality. Such production was achieved only because of the building tack provided by undertread cements, and, prior to the present invention, the tire industry was not aware that it could be achieved without such cements and did not appreciate the importance of eliminating undertread cements.
Heretofore rubber compounders have not been concerned about the degree of unsaturation of C.sub.16 -C.sub.18 fatty acids sold under the name "stearic acid" and have routinely employed fatty acid mixtures containing from 25 to 50% by wt of oleic acid. In some instances, mixtures containing high proportions of oleic acid were preferred to minimize problems due to calcium stearate bloom.
Because rubber compounders heretofore were not fully aware of and concerned about the effects of various compounding ingredients on the tack and surface characteristics of extruded rubber treads, it was common practice to employ ingredients which significantly interfere with tack and tack retention. For example, it was common to employ excessive amounts of ingredients which can adversely affect the surface due to diffusion or migration, such as cyclohexyl-n-thiophthalimide, diphenyl-p-phenylenediamine and wax and other antiozonants.
Heretofore the standared equipment in a tire factory for processing treads for radial tires included a take-away belt conveyor for moving the dual-layer tread strip from the extruder to the so-called "shrink rolls," a conveyor for moving the tread strip past a weighing station, and a cementing station with rolls for receiving and supporting the strip while applying the undertread cement. It is necessary to support the tread strip with the tread cap in the top position and to keep the conveyor belts and the undertread surface dry and free of water until the weighing and cementing operations are completed.
In a typical plant the tread strip travels 50 to 60 feet (ft) from the extruder to the cementing station and is then carried 20 to 60 ft or more to a large cooling tank containing belt conveyors arranged in several tiers to support a tread strip with a length of several hundred ft. A large number of water sprays are provided in the tank above each section of the belt conveyor to direct water sprays down against the tread cap. The water spray provides a high rate of heat transfer; but, because of the notoriously poor heat conductivity of rubber, each portion of the tread strip leaving the extruder at 120.degree. C. may remain at a temperature above 95.degree. C. for several minutes depending on the rubber thickness and the rate of extrusion which for some treads is below 60 ft/min.
In a typical plant, each portion of the tread strip travels about 200 ft to the second tier of the cooling tank before there is rapid cooling of the undertread by the water sprays. During such travel, which usually takes several minutes, the surface temperature of the undertread is considerably higher than that of the tread cap. The water which is applied to the conventional conveyor belts carrying the cemented tread strip to the cooling tank has a cooling effect, but such incidental cooling of the undertread does not reduce the surface temperature significantly.
The normal tread cooling procedures of the type described above have been used for more than a decade. They cause the surface of the undertread to remain hot longer than the outer surface of the tread cap and promote undesirable diffusion and migration of zinc stearate and other ingredients to the undertread surface. The tire industry did not recognize this and did not understand how the standard procedures or slow extrusion rates could adversely affect building tack. Also the rates of extrusion were limited by the nature of the existing equipment and the cooling capacity of the cooling tank. Such limitations in existing factories, the cost of equipment and other practical considerations prohibited major changes in the procedures for extruding and cooling the treads.
The prior art procedures were such that undertread cements were essential to obtain adequate building tack, and those skilled in the art did not see a reason to change those procedures and had no reason to believe that it was feasible to eliminate the undertread cement in the standard factory process. For example, a change in a conventional cooling tank to increase the rate of cooling from 10 to 20% would be prohibitive because of cost and ineffective because applied in the wrong place and too late to have any effect on tack. Prior to this invention the tire industry saw no way to save significant amounts of money in manufacturing its tires by investment in new cooling equipment.